C99 integer constants with no type suffix promote differently
depending on the way that they're written: hex and oct consts
promote as int => uint => long => ulong => vlong => uvlong.
Decimal constants are always signed.
We used to promote all values to uint on overflow, and never
went wider. This change fixes that, and adds a warning when
a decimal constant that would have been promoted to uint in
the past gets promoted to int.
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